As an optical recording medium, various kinds have been proposed, for example, read-only type optical discs such as CD (compact disc) and LD (laser disc), and writable optical discs such as MO (magnetic optical disc) and MD (mini disc), in which a guide groove for tracking and a pattern for discrete information as a pre-format are preformed.
Manufacturing methods of such various optical discs generally include a step of manufacturing a metal master called a stamper which has a surface shape corresponding to a desired concavo-convex pattern of pits, a groove, and the like (a master manufacturing step), a step of molding for transferring the surface shape of the stamper onto a disc substrate, and a step of forming certain layers such as a recording layer and a protective layer.
Of these steps, in the master manufacturing step, a procedure of washing and drying a glass substrate having polished surfaces, applying photoresist, which is a sensitive material, on the glass substrate, and emitting the optical beams such as laser beams to the photoresist to form the pattern of the pits and the groove, is generally performed. A latent image formed on the photoresist by the above exposure is developed to form a concavo-convex pattern corresponding to the three-dimensional pattern of the pits or the groove on the photoresist, the pattern is further transferred onto a surface of a metal by the electroforming to produce the stamper. The procedure of recording the concavo-convex pattern using the exposure with such optical beams requires faithfully transferring certain pattern onto the photoresist surface in precision of the sub micron order.
On the other hand, a technique has been proposed for using a groove itself as a record track in the read-only type optical discs such as CD and LD and the rewritable type discs such as MD and MO. In these optical recording media, while the optimal pit width and the optimal groove width depend on the kinds of the media, and any of the masters for manufacturing optical recording medium are manufactured by the above master manufacturing methods, so the pit width and the groove width are defined by the spot diameter of the exposure beams.
An aligner having a schematic structure as shown in FIG. 5 has conventionally been used for recording on the optical recording media like CD and LD using one exposure beam. The aligner comprises a gas laser light source 101 which uses gas as an amplification medium like a He-Cd laser, a mirror 103 which leads a laser light 102, which is emitted from the gas laser light source 101, towards a latter optical system, a beam condenser 104 which condenses (reduces) the laser light 102 being led through the mirror 103, an AOM (Acousto Optical Modulator) 105 which modulates the optical intensity of the beam being controlled by a driver 115 corresponding to ultrasonic waves modulated and supplied based on record signals, a beam-diameter regulation lens 106 which enlarges or reduces the beam diameter of the laser light 102 being intensity-modulated by the AOM 105, a moving optical table 108, and a mirror 107 which reflects the beam, which is emitted from the beam-diameter regulation lens 106 and goes straight in a form of a collimated beam, and horizontally passes it onto the moving optical table 108.
Furthermore, on the moving optical table 108 are arranged a third lens 109, a mirror 110, and an objective lens 111. The third lens 109 is arranged in a position which makes the beam of the laser light 102 gather on an incident side condensing surface 113 which is formed in a position conjugated with an imaging condensing surface 112 of the objective lens 111. An exposure beam is directed to a photoresist film 114 via the third lens 109 of the moving optical table 108, the mirror 110, and the objective lens 111.
In addition, the spot diameter of the exposure beam can be adjusted by changing a focal distance of the beam-diameter regulation lens 106 or the third lens 109 being positioned between the gas laser light source 101 and the objective lens 111, and further readjusting an effective numerical aperture NA of the objective lens 111 so that the objective lens 111 may condense the exposure beam on a surface of the photoresist film 114.
On the other hand, for example, in the MO discs, the technology of recording pits and a groove with two exposure beams has been proposed, for example, in Japanese Laid Open No. 06-103613. As shown in FIG. 6, in an optical recording apparatus, on the same optical axis are arranged one laser light source 201, a beam splitter 203 which divides a beam of laser light 202 being emitted from the laser light source 201 into two beams, a reflective mirror 204, a beam relay optical system 205, a beam relay optical system 206, and a moving optical table 208 equipped with a PBS (polarization beam splitter) 207 which regenerates the two divided laser beams.
The laser light 202 being emitted from the laser light source 201, goes straight keeping a form of a collimated beam, and is divided into reflected light (S polarized light) 209 and passing light (S polarized light) 210 by the beam splitter 203. The passing light 210 is reflected from the reflective mirror 204 and directed onto the beam relay optical system 206, and the reflected light 209 is directly led to the beam relay optical system 205. The passing light 210 is condensed on a first AOM 212 with a condenser lens 211 within the beam relay optical system 206. The first AOM 212 is controlled by a driver 214 and modulates the intensity of the beam of the passing light 210. The beam which is intensity-modulated by the first AOM 212 is reflected from a lens 213 to become a collimated beam of P polarized light. The collimated beam is polarized by a λ/2 polarizing plate 215 to become a collimated beam 216 of S polarized light.
On the other hand, the reflected light 209 becomes a collimated beam 221 of S polarized light through the beam relay optical system 205 comprising a condenser lens 217, a driver 218, a second AOM 219, and a lens 220.
The collimated beam 216 is reflected from a mirror 222, the collimated beam 221 is reflected from a mirror 223, and both are horizontally directed to the moving optical table 208 in parallel. In the moving optical table 208, the collimated beam 216 passes directly through the PBS 207. On the other hand, the collimated beam 221 is reflected from a mirror 224 to shift the traveling direction 90 degrees away from the original direction to be directed to the PBS 207, reflected inside the PBS 207, and aligned on the optical axis of an objective lens 226 through a lens 225 and a reflective mirror 227. As described above, the two collimated beams 216 and 221 which enter the PBS 207 are combined with the PBS 207. Here, a reflective surface of the PBS 207 is set so that the traveling direction of the combined and emitted laser beams may have a moderate angle of reflection. Thus, when the laser beams being emitted from the PBS 207 are condensed on a surface of a photoresist film 228 which is an exposed object and is parallel to an imaging condensing surface of the objective lens 226, spots of the two collimated beams 216 and 221 can be condensed respectively in positions which are shifted on the order of sensitivity and resolution of the photoresist film 228 and pits are exposed in the nearly center between adjacent parts of the groove in the radius direction.
Using such exposure method, a latent image of pits and a groove based on MO formats can be exposed (recorded). Furthermore, a latent image of pits and a groove based on various formats such as single density MO, double density MO, and quad density MO can be exposed (recorded) by changing focal distances of the condenser lenses 211 and 217 and the lens 225, readjusting so that the objective lens 226 may condense beams on the surface of the photoresist film 228, and changing the diameter of the spots of the exposure beams.
Incidentally, it is confirmed that there is possibility that pits with an U-shaped cross section and wobbling grooves shallower than the depth of the pits may become a leading technology corresponding to increased high density for realizing increasing the storage capacity thereof in optical recording media such as DVD-RW (digital versatile disc-rewritable) upon which practice of high density recording is imposed. DVD are generally set to perform recording on flat parts which can secure predetermined amounts of record signals, for example, on the bottoms of the grooves and on the upper surfaces of lands, and it is confirmed that effective recording and reproduction of information are possible, even if recording parts are located in a comparatively shallow position.
When such master for manufacturing optical recording medium is produced, pits 301 are exposed so that espousing light for a photoresist film 302 may be deep enough to reach even a surface of a substrate 303 as typically shown in FIG. 7, so the cross section thereof has a U-shaped deep hole with a flat bottom 304.
However, in order to form a groove 305 shallower than the pits 301, light exposure for exposing the photoresist film 302 is purposely reduced to stop the exposing at a shallow place on the way without extending to a deep position in the thickness direction of the photoresist film 302 during producing the master for manufacturing optical recording medium, but a three-dimensional pattern of the shallower groove 305 obtained by developing a latent image formed with such lowered exposure light has a cross section of a V-shaped groove, and the width Wg of the groove 305 (the width in the radius direction of the disc-like substrate 303) narrows down to half or less of the width Wp of the pits 301, which results in a problem that sufficient amounts of servo signals (amounts of PUSH-PULL signals) at the time of reproduction or the like of the optical recording media manufactured from such master can not be obtained. Moreover, the V-shaped cross section of the groove 305 increases deviation (dispersion) of distribution of the depth dg of the groove, which results in a problem that recording and reproduction properties are lowered. Moreover, in the manufacturing process of the optical recording medium, laminating and forming furthermore a reflective film, a protective layer, and the like on a surface of the shallow groove with the V-shaped cross section can further reduce the substantial width of the groove 305.
And, in order to solve these problems, it is considered that there is also a method of increasing the light exposure to obtain certain width in the step of exposing the shallow groove 305 which is the factor thereof, but such increase of light exposure makes the exposure reach a deep position in the thickness direction of the photoresist film 302, and it is significantly difficult to form the groove 305 having the U-shaped cross section with certain width and shallower than the pits 301 in the conventional technique, for example, it is impossible to form even the groove 305 shallower than the pits 301.
The present invention has been achieved in view of the above problems. It is an object of the invention to provide an optical recording medium enabling high density recording, where a groove having U-shaped cross section being shallower than pits and the like and being wider than certain width is formed, a master for manufacturing optical recording medium, a manufacturing apparatus for manufacturing the master and a manufacturing method thereof.